8. BACK PRESSURE.
Turbo manifold back pressure is another
area we should consider also, as it can contribute to many problems and low power output
as well. If we have increased the boost and are now forcing in much more air and fuel than
normal, it figures that it has to escape in much larger volumes now especially as it has
vastly increased in volume through the combustion process. All this gas is trying to
escape through the same small hole in the turbo snail with that same small exhaust exducer
wheel in the way.
Needless to say the forces acting on the
wheel turn it faster and make more boost which is good, but can be bad if the gas cannot
escape quickly enough. The pressure in the exhaust manifold can be more than the intake
manifold!
This is DOES NOT lead to efficient operation. If you have upgraded your exhaust system and
are not running over 1 BAR (14.7 PSI) then you probably don't have to worry about it. On
the other hand if you are running much more boost (probably an intercooler as well) then
it may be worthwhile to check if you have not modified your exhaust exducer wheel by back
cutting or increasing in size. (or both, check with turbo expert
vendors on your exact requirements.)
The TurboClub can only advise in
general terms as your vehicle is probably unique in many ways, and deserves expert advice
to progress beyond the basics.
9. FUEL Enrichment
The problem with increasing the boost too high, is the injectors are driven by the
computer to near one hundred per cent duty cycle. Or in other words, open as much as possible
to supply the maximum amount of fuel.
Or the fuel maps in the computer wont actually drive the injectors to near 100%.
Once this limit is reached the the fuel air mixture then starts to lean out as the boost
and/or air volume increases past the computers and injectors capacity to compensate. There
are a few ways to add extra fuel to overcome this situation that involve various levels of
complexity and cost.
1. increase fuel pressure above normal at 100% duty cycle of injectors.
2. add extra fuel injector and switch on at preset boost level.
3. replace fuel injectors with larger capacity size.
4. replace computer or ROM chip to drive injectors to 100%
5. All of the above as I have done for very high boost (up to 25 PSI)
